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While the human body has sufficient stores of fat to run vast distances, the supply of carbohydrate is quite limited. This carbohydrate store is in the form of Glycogen, a branching chain of glucose molecules.
* Burning glycogen for energy requires less oxygen than fat, making it more efficient. However, the store of glycogen is limited, and when the supply runs low, we "hit the wall".
* Glycogen is stored primarily in the muscles, but that glycogen can only be used by the muscle it's stored in and cannot flow from the muscles through the blood to other places. The glycogen is stored within a muscle fiber, not the overall muscle, so when a fiber gets glycogen depleted, it can't use glycogen from surrounding fibers.
* Some glycogen is stored in the liver where it flows through the blood to all tissues. The human liver typically stores between 90 and 160 grams of Glycogen, or 350 to 650 Calories.
* Blood typically contains less than 20 calories of glucose. (This assumes 5 liters of blood and 100mg/dL of blood glucose, which is 5g of glucose.)
* Eccentric exercise, such as [[Downhill Running]], can cause [[Delayed Onset Muscle Soreness| DOMS]] and impair glycogen replenishment<ref name="O'Reilly-1987"/>.
* Glycogen stores may not be replenished between daily hard runs, such as 10 miles at 80% of [[VO2max|V̇O<sub>2</sub>max]]<ref name="Costill-1971"/>.
* Each gram of Glycogen is stored with between 3-4g of water<ref name="OlssonSaltin1970"/>. This means you can lose a lot of weight rapidly after long runs or brief diets, but this is mostly water weight. This water can provide hydration, offset sweating or increasing urine volumes. Consequently, you may need less fluid earlier in a run compared with later.
* [[High Intensity Interval Training]] may deplete glycogen reserves rapidly. This is anerobic exercise only produces 1/15 the energy from glycogen, so the typical 2,000 calories energy reserve would only give 133 calories!
=Glycogen Usage=